Nervous Tissue: Introduction and Overview

Introduction to the Nervous System

The nervous system is a complex network responsible for receiving, processing, and responding to internal and external stimuli. It consists of various organs, including the brain, spinal cord, sensory receptors (such as those in the eyes and ears), and nerves that connect the nervous system to other body systems.

• Nervous tissue forms the main component of these organs, supported by blood vessels and connective tissues.

• The basic functional units are neurons, specialized for intercellular communication.

• Neuroglia (or glial cells) support, protect, and nourish neurons.

Anatomical Divisions of the Nervous System

Central Nervous System (CNS)

The CNS consists of the brain and spinal cord. These organs integrate, process, and coordinate sensory data and motor commands.

• Sensory data: Information about internal and external conditions.

• Motor commands: Control or adjust peripheral organs (muscles, glands).

• Protected and supported by blood vessels and connective tissues.

Peripheral Nervous System (PNS)

The PNS includes all nervous tissue outside the CNS and the enteric nervous system (ENS).

• Delivers sensory information to the CNS and carries motor commands to peripheral tissues.

• Nerve fibers (bundles of axons) transmit sensory and motor signals.

• Peripheral nerves are nerve fibers with associated blood vessels and connective tissues.

Functional Divisions of the Nervous System

Afferent and Efferent Divisions

The PNS is functionally divided into afferent (sensory) and efferent (motor) divisions.

• Afferent division: Brings sensory information from receptors to the CNS.

• Efferent division: Carries commands from the CNS to effectors (muscles, glands, adipose tissue).

Subdivisions of the Efferent Division

• Somatic Nervous System (SNS): Controls voluntary and involuntary (reflex) skeletal muscle contractions.

• Autonomic Nervous System (ANS): Regulates smooth muscle, cardiac muscle, glands, and adipose tissue at the subconscious level.

• The ANS is further divided into the parasympathetic (rest-and-digest) and sympathetic (fight-or-flight) divisions, which often have opposite effects.

Structure of Neurons

General Features

• Neurons are generally long-lived, have a high metabolic rate, and are typically unable to divide.

• They possess an excitable plasma membrane capable of generating and propagating action potentials.

Major Components of a Typical Neuron

• Cell Body (Soma/Perikaryon): Contains the nucleus, mitochondria, free and fixed ribosomes, and rough endoplasmic reticulum (RER). Clusters of RER and ribosomes are called Nissl bodies.

• Dendrites: Branched extensions that receive signals from other neurons.

• Axon: A long process that propagates action potentials. The axon hillock is the initial segment connecting the axon to the cell body.

• Telodendria: Terminal branches of the axon ending in synaptic terminals.

Axoplasmic Transport

• Materials move between the cell body and axon terminals via axoplasmic transport.

• Anterograde flow: From cell body to axon terminal.

• Retrograde flow: From axon terminal to cell body.

• Example: The rabies virus uses retrograde flow to reach the CNS.

Classification of Neurons

Structural Classification

• Anaxonic neurons: Small, with many dendrites and no obvious axon. Found in the brain and special sense organs.

• Bipolar neurons: One dendrite and one axon with the cell body between them. Found in special sense organs (retina, olfactory epithelium, inner ear).

• Unipolar (pseudounipolar) neurons: Dendrites and axon are continuous, with the cell body off to one side. Most sensory neurons in the PNS are unipolar.

• Multipolar neurons: Multiple dendrites and a single axon. Most common in the CNS and all motor neurons controlling skeletal muscles.

Functional Classification

• Sensory (Afferent) Neurons: Carry sensory information from receptors to the CNS. Cell bodies are located in peripheral sensory ganglia. Most are unipolar.

• Motor (Efferent) Neurons: Carry instructions from the CNS to effectors. Somatic motor neurons control skeletal muscles; visceral motor neurons control smooth muscle, cardiac muscle, glands, and adipose tissue.

• Interneurons (Association Neurons): Located mainly in the CNS, they connect sensory and motor neurons and are involved in higher functions such as memory, planning, and learning.

Types of Sensory Receptors

• Interoceptors: Monitor internal systems (digestive, respiratory, cardiovascular, urinary, reproductive) and provide sensations of distension, pressure, and pain.

• Exteroceptors: Provide information about the external environment (touch, temperature, pressure, special senses).

• Proprioceptors: Monitor position and movement of skeletal muscles and joints.

Neuroglia (Glial Cells)

Introduction and Functions

Neuroglia are supporting cells that separate and protect neurons, provide a supportive framework, act as phagocytes, and regulate the composition of interstitial fluid. They outnumber neurons and are essential for nervous system function.

Neuroglia in the CNS

• Astrocytes: Star-shaped, most numerous. Functions include maintaining the blood-brain barrier (BBB), providing structural support, repairing damaged tissue, guiding neuron development, and regulating the interstitial environment.

• Ependymal Cells: Line the ventricles of the brain and central canal of the spinal cord; produce and circulate cerebrospinal fluid (CSF).

• Oligodendrocytes: Myelinate axons in the CNS, providing electrical insulation and increasing the speed of action potential conduction. One oligodendrocyte can myelinate multiple axons.

• Microglia: Smallest and least numerous; act as phagocytic cells, removing debris, waste, and pathogens. They are the resident immune cells of the CNS.

Neuroglia in the PNS

• Satellite Cells: Surround neuron cell bodies in ganglia, regulating the environment around neurons (similar to astrocytes in the CNS).

• Schwann Cells (Neurolemmocytes): Myelinate axons in the PNS. Each Schwann cell myelinates one segment of a single axon. Non-myelinating Schwann cells can enclose multiple unmyelinated axons.

Myelination and Nervous Tissue Organization

Myelin Sheath

• Myelin is a membranous wrapping that insulates axons, increasing the speed of electrical signal transmission.

• In the CNS, oligodendrocytes myelinate axons; in the PNS, Schwann cells perform this function.

• Myelinated axons appear glossy white (white matter), while regions with neuron cell bodies, dendrites, and unmyelinated axons appear gray (gray matter).

• Gaps between myelinated segments are called nodes of Ranvier.

Comparison Table: CNS vs. PNS Neuroglia

Demyelination and Nervous System Disorders

Demyelination

• Demyelination is the progressive destruction of myelin sheaths in the CNS and/or PNS, leading to loss of sensation and motor control.

• Causes include heavy metal poisoning (arsenic, lead, mercury), infections (e.g., diphtheria), and autoimmune diseases (e.g., multiple sclerosis).

• Multiple Sclerosis (MS): An autoimmune disorder causing demyelination in the CNS, leading to neurological deficits such as muscle weakness, vision problems, and impaired coordination.

Key Terms and Definitions

• Nissl bodies: Clusters of rough endoplasmic reticulum and ribosomes in neuron cell bodies, involved in protein synthesis.

• Ganglia: Collections of neuron cell bodies in the PNS.

• Sensory neuron: Neuron that transmits sensory information to the CNS.

• Motor neuron: Neuron that transmits commands from the CNS to effectors.

• Anterograde flow: Movement of materials from the cell body to the axon terminal.

• Retrograde flow: Movement of materials from the axon terminal to the cell body.

• White matter: Regions of the CNS with myelinated axons.

• Gray matter: Regions of the CNS with neuron cell bodies, dendrites, and unmyelinated axons.

• Demyelination: Loss or destruction of myelin sheaths.

Summary Table: Neuron Types by Structure and Function

Additional info:

• Neurons communicate via synapses, where neurotransmitters are released to transmit signals to other neurons or effectors.

• Reflexes are rapid, involuntary responses mediated by the nervous system, often involving interneurons.

• Myelination increases the speed of action potential propagation via saltatory conduction.